1. Field of the Invention
This invention relates generally to registration apparatus for screen printing machines, and more particularly to a registration system for accurately registering multiple silkscreens after they have been clamped into the silkscreen holders of the printing heads of a screen printing machine, and for preparing an image bearing silkscreen to be used therein.
2. Brief Description of the Prior Art
The present invention will be described in connection with, although not limited to, a rotary or carousel screen printing machines, which print on cloth, paper, plastic and other products. These machines have a plurality of flat generally rectangular platens or pallets secured to the outer ends of radial arms mounted on a common turntable rotatable in a path beneath an array of printing stations. Each printing station contains a printing head having a silkscreen frame holder for holding the silkscreen frame. The silkscreen frame has stretched across it the screen material upon which is an exposed image. Typically, the silkscreen frame holder comprises a pair of opposed C-shaped channels which receive the silkscreen frame and a pair of pneumatic clamp members on each channel that lock or clamp the frame into the holder. The printing head may also have an ink dispenser that dispenses ink at one end of the silkscreen and a squeegee that is pulled across the silkscreen to evenly apply the ink. The printing station or printing head may also be provided with heating, drying and other well known structural components. Most commercial printing heads are also provided with micro-adjustment mechanisms to allow small vernier displacements to shift the silkscreen frame a few thousandths of an inch.
Newman et al, U.S. Pat. No. 5,771,801 and Sundqvist, U.S. Pat. No. 5,315,929 disclose various micro-adjustment mechanisms for registering silkscreens.
The pallets support the articles to be printed and are advanced around the printing stations and positioned underneath the silkscreen frame holders of the printing heads, brought into contact with the silkscreens, and a pattern or image of the appropriate color is applied and "squeegeed" through the silkscreen onto the article. Typically, there are two more platens than the number of printing heads so that articles to be printed may be simultaneously placed on one and removed from another without interference from components at the printing heads. In this arrangement, each of the silkscreens in the sequence commonly prints a different image on top of the previously printed image, and this subsequent image is of a different color and design.
It has been recognized that the more consistent the alignment of each image on the screen mesh in each of the silkscreen frames, the less time and effort that is typically required to precisely align the screen printing frames within the printing machine to arrive at production printing. In creating a silkscreen to be used on automated machines of the type described above, a one color graphic design or image is photographically transferred to a piece of film called a film positive, the image being black, the rest of the film being clear.
A common process for creating and applying the artwork or image to the silkscreen is known as a "pin registration system", which utilizes a light table, a pin bar, an exposure table, a rectangular exposure frame, and pin-registered films known as "carrier sheets". A pin bar is secured at one end of a light table. The pin bar is a thin rectangular strip of material having a set of laterally spaced pins projecting upwardly therefrom for receiving corresponding holes punched in a transparent "carrier sheet". The light table may have a grid or a series of lines for assisting in the lay-up. A film positive of the first image to be printed in a first color is aligned and secured to the carrier sheet. The second carrier sheet having holes punched in it is placed on the pin bar. A film positive of the image to be printed in a second color is placed on the second carrier sheet and precisely aligned in superposed relation over the first image and then secured to the carrier sheet. This process is repeated as necessary using subsequent carrier sheets and film positives of the subsequent images to be printed in different colors.
A thin flat generally rectangular exposure frame is secured to the glass top of an exposure table. The exposure frame has a set of pins protruding upwardly from one edge that match the pins of the pin bar, and receive the holes of the carrier sheets having the film positives secured thereto. The exposure frame also has a pair of push pins or stop blocks longitudinally spaced on one side and a third push pin or stop block on a second side that face the central opening and engage two sides of the frame of the silkscreen. A carrier sheet having a film positive of the first image to be printed in a first color is placed on the pins of the exposure frame. A silkscreen coated with a light-sensitive, photochemical translucent emulsion is placed in the exposure frame and two of its sides pushed against the push pins or stop blocks on the two sides of the exposure frame. The vacuum blanket of the exposure table is lowered onto the silkscreen frame and a vacuum is drawn to ensure that there no movement between the carrier sheets and screen mesh. The image is than exposed to a bright light and photochemically developed, or "burned-in", onto the screen. Thus, the image portion of the film positive is burned through the emulsion, leaving that portion of the screen mesh open and porous, while the non-image areas of the film positive will have no effect on the emulsion, thus leaving it on the screen. This process is repeated as necessary using subsequent carrier sheets and film positive images for the various different colors. The exposed silkscreens are washed in a known manner and are then ready to be mounted in the print heads of the printing machine.
Hoffman et al, U.S. Pat. No. 5,129,155, Newman, U.S. Pat. No. 5,648,189, and Winter, U.S. Pat. No. 5,664,495 disclose various apparatus for accurately aligning an image on a silkscreen when the image is transferred to the screen.
It is necessary to preposition, or align the screens before printing, so that, when each screen sequentially prints its portion of the composite print graphic, the images are in the approximate exact positions to accurately reproduce and reflect the original artwork. This process of prepositioning, or aligning multiple screens is called "registering", and before any production run utilizing multiple screens can begin, all the screens must be "registered", to ensure the proper fit of colors. Some designs may require colors to be separated by a required distance, while others may require colors to be aligned edge to edge with no separations. If accurate registration is not accomplished, colors may overlap and "bleed" into one another, or alternatively, be separated when they should be touching. Prints that are "out of register" may be blurry, inaccurate and generally inconsistent with the original artwork.
Numerous methods and systems for prepositioning, or registering silkscreens on a printing apparatus have been known and practiced throughout the history of the art.
In a method commonly used, a film positive of the first piece of artwork or image that was used to create the silkscreen is secured onto one of the pallets and the pallet is rotated beneath the first printing head. A silkscreen having that same piece of artwork or image burned into it is placed into the silkscreen holder of the first printing head. Typically, the first piece of artwork is left in place and all subsequent silkscreens are registered to that same first piece of artwork. The pallet is raised up, and then observing the film positive through the translucent emulsion on the silkscreen, the operator visually aligns the image on the silkscreen with the artwork on the pallet and manipulates the silkscreen manually until the artwork on the screen is aligned with the artwork on the pallet.
The silkscreen frame is then locked into the silkscreen frame by actuating the pneumatic clamps. However, the actuation and clamping action of the clamps will nearly always cause the silkscreen frame to shift slightly, and thus become out of registration. After the silkscreen frame has been locked down by the clamps, and again observing through the translucent emulsion on the silkscreen, the knobs of the micro-registration mechanism are manually turned to move the silkscreen frame holder and silkscreen to adjustably fine-tune the alignment of the silkscreen image to the artwork on the pallet.
After visual registration, the micro-adjustment mechanism is locked down. Then the pallet having the first piece of artwork or image secured to it is lowered and rotated beneath the next printing head having a silkscreen with the next piece of artwork or image for the next color, and the visual registration, clamping, and micro-adjustment process is repeated again, and again with all of the subsequent screens for the subsequent pieces of artwork or images to be reproduced in different colors.
After all of the silkscreens have been visually registered, the printing cycle is then started, and a trial and error procedure is used. A test article is printed, and the results are analyzed to determine necessary screen adjustments. The screen adjustments are accomplished by attempting to analyze which of the silkscreens are out of registration, and then repeating the visual registration, clamping, and micro-adjustment process for the suspected misaligned silkscreens. This time consuming inefficient procedure generally may be repeated several times depending on luck and experience of the printer, thereby increasing the cost because the printing machine is non-operative. Also, this method requires experienced operators or operators with patience to set up the machine for each job. Even with the micro-registration mechanisms, this process requires visual alignment observing the superposed images through the silkscreen having the translucent emulsion on its surface.
Although, the micro-adjustment mechanisms have improved the set-up process, they do not eliminate the cumulative error due to the placement of the silkscreen frames in the silkscreen frame holders and the positioning of the silkscreen frame and image to the printing surface, and have no relationship to the placement of the image on the silkscreen when the image is applied to the silkscreen.
Schlife et al, U.S. Pat. No. 5,226,366 discloses an apparatus and method for centering a silkscreen on a platen and wherein the silkscreen may be aligned on the printing machine using the same or different apparatus by contacting the screen with registration members at the same points of contact used when the screen was aligned for application of the artwork.
Most prior art registration and micro-adjustment devices require visual alignment looking through the screen mesh or visual alignment of an index mark, and do not provide visual numerical indicators that will let the operator know that the silkscreen has shifted, which silkscreen has shifted, or how much it has shifted, and are not capable of providing settings which may be used to re-register the artwork and place the silkscreen frame in its original registered position.
The present invention is distinguished over the prior art in general, and these patents in particular by a system for accurately registering multiple silkscreens relative to a printing surface after they have been clamped into a silkscreen holder of a printing head of a rotary screen printing machine, and for accurate placement of an image on a framed silkscreen. A generally L-shaped registration device attached adjacent to one side of a pallet of the printing machine has first and second sides orthogonal relative to each other with a pair of gauges having visual displays positioned on the first side and a third gauge positioned on the second side. The gauges have outwardly biased plungers controlled by a lever to be simultaneously moved between a retracted positioned, a free-floating position and an engaged position relative to respective first and second sides of a silkscreen frame. The gauge displays allow the operator to take numerical readings to determine if, and how much, a frame has shifted from a zero setting after being clamped in the frame holder, which frame has shifted, to quickly re-register a frame to its previous setting, and to compensate for frames having misaligned images. Another aspect of the invention is an exposure frame for use in conjunction with the registration device having three contacts corresponding to the plunger positions of the registration device that engage a framed silkscreen during exposure of the image to create silkscreens having images aligned relative to the registration frame.